resolvergen: use the resolver type as base name for dependent types

The template was outputing invalid code since the resolver type was
not used in places like the embedding at {query,mutation}Resolver.

This change also ensures that objects like {query,mutation}Resolver
also use the user provided type name as suffix.

Here's the resulting diff on the code generation with `type:
GeneratedResolver` in the resolver config:

```
diff -u resolver.go resolvernew.go
--- resolver.go 2019-05-26 20:04:15.361969755 -0300
+++ resolvernew.go      2019-05-26 20:04:54.170737786 -0300
@@ -7,20 +7,20 @@
 type GeneratedResolver struct{}

 func (r *GeneratedResolver) Mutation() MutationResolver {
-       return &mutationResolver{r}
+       return &mutationGeneratedResolver{r}
 }
 func (r *GeneratedResolver) Query() QueryResolver {
-       return &queryResolver{r}
+       return &queryGeneratedResolver{r}
 }

-type mutationResolver struct{ *Resolver }
+type mutationGeneratedResolver struct{ *GeneratedResolver }

-func (r *mutationResolver) CreateTodo(ctx context.Context, input NewTodo) (*Todo, error) {
+func (r *mutationGeneratedResolver) CreateTodo(ctx context.Context, input NewTodo) (*Todo, error) {
        panic("not implemented")
 }

-type queryResolver struct{ *Resolver }
+type queryGeneratedResolver struct{ *GeneratedResolver }

-func (r *queryResolver) Todos(ctx context.Context) ([]*Todo, error) {
+func (r *queryGeneratedResolver) Todos(ctx context.Context) ([]*Todo, error) {
        panic("not implemented")
 }
```

plugin/resolvergen/resolver.gotpl

@@ -20,18 +20,18 @@ type {{.ResolverType}} struct {}
 {{ range $object := .Objects -}}
 	{{- if $object.HasResolvers -}}
 		func (r *{{$.ResolverType}}) {{$object.Name}}() {{ $object.ResolverInterface | ref }} {
-			return &{{lcFirst $object.Name}}Resolver{r}
+			return &{{lcFirst $object.Name}}{{ucFirst $.ResolverType}}{r}
 		}
 	{{ end -}}
 {{ end }}
 
 {{ range $object := .Objects -}}
 	{{- if $object.HasResolvers -}}
-		type {{lcFirst $object.Name}}Resolver struct { *Resolver }
+		type {{lcFirst $object.Name}}{{ucFirst $.ResolverType}} struct { *{{$.ResolverType}} }
 
 		{{ range $field := $object.Fields -}}
 			{{- if $field.IsResolver -}}
-			func (r *{{lcFirst $object.Name}}Resolver) {{$field.GoFieldName}}{{ $field.ShortResolverDeclaration }} {
+			func (r *{{lcFirst $object.Name}}{{ucFirst $.ResolverType}}) {{$field.GoFieldName}}{{ $field.ShortResolverDeclaration }} {
 				panic("not implemented")
 			}
 			{{ end -}}